I have found throughout my life that, if all else fails, the character of a man can be recognized by nothing so surely as by a jest which he takes badly.

Georg C. Lichtenberg (1742-1799) German physicist, writer

Aphorisms, Notebook K, #46 (1793-96) [tr. Hollingdale (1990)]
    (Source)

See also Goethe. Alternate translation: "A person reveals his character by nothing so clearly as the joke he resents."